Electrolytic production of metallic fluoborates



$8111.24, 1967 G. BALTAKMENS ET AL 3,300,397

ELECTROLYTIC PRODUCTION OF METALLIC FLUOBORATES Filed March 14, 1963 HBF4 LAYER H02 LAYER INVENTORS: GOTLIBS BALTAKMENS JOHN TOURISH AT TO R N E Y United States Patent 3,300,397 ELECTROLYTIC PRODUCTION OF METALLIC FLUOBORATES Gotlibs -Baltakmens, Wilmington, Del., and John P. Tourish, Swarthmore, Pa., assignors to Allied Chemical Corporation, New York, N.Y., a corporation of New York Filed Mar. 14, 1963, Ser. No. 265,211 7 Claims. (Cl. 204-94) This invention relates to the electrolytic production of metallic fiuoborates and, more particularly, to the production of metallic fluoborates such as tin, copper and nickel fluoborates by electrolytic dissolution of the appropriate metal in fiuoboric acid.

Although the invention is hereinafter exemplified by the production of stannous fluoborate, it should be understood that it is equally applicable to the production of other metallic fiuoborates.

Stannous fluoborate solution is commercially produced by reacting stannous oxide with hydrofluoric acid and then adding boric acid to form the corresponding fiuoborate. The oxide is used as the source of tin since, under normal reaction conditions, elemental tin does not dissolve in either hydrofluoric or fiuoboric acid. There would be a decided price advantage in using tin instead of stannous oxide; hence, electrolytic methods for dissolution of tin in fiuoboric acid have been tried. In such procedures, tin constitutes the anode while the cathode is any suitable electrode material. The tin anode dissolves upon application of electric current, thereby producing stannous fluoborate at the anode and hydrogen which is evolved at the cathode.

U.S.P. 2,673,837 of March 30, 1954, discloses production of metallic fiuo-borates by use of an electrolytic cell having horizontally disposed solid electrodes, the anode being near the bottom and the cathode being near the top of the cell. Fluoboric acid solution is introduced near the top, and metallic fluoborate solution is withdrawn near the bottom of the cell. When this type of cell is employed using copper tubing as cathode, free metal deposits on the cathode and causes treeing in the form of an extremely loose sponge. All attempts at automatic removal of the sponge have been unsuccessful. The reason for the. lack of success resides in the physical nature of the sponge. While it is readily removed, it also compacts easily to a dense, hard plate. Automatic scraping, while removing the greater part of the sponge, has compacted a small amount with each operation. This compacted mass then grows until it jams the device or breaks the scraper itself.

An object of the present invention is to provide an improved process for the electrolytic production of metallic fluoborates.

It is a further object of the invention to provide an improved continuous process for the production of metallic fluoborates, wherein recirculating mercury is employed as cathode.

A still further object of the invention is to provide apparatus for carrying out these processes.

Other objects and advantages of the invention will be apparent from the following description.

In accordance with this invention, metallic fiuoborates such as stannous fluoborate are produced by introducing fiuoboric acid solution in an electrolytic cell provided with an anode of the metal to be converted into its fluoborate near the bottom of the cell and a mercury cathode near t-he top of the cell, the mercury cathode being maintained in a container provided with a porous woven bottom, and withdrawing the metallic fluoborate so produced from the vicinity of the anode. h

3,300,397 Patented Jan. 24, 1967 As the stannous fluoborate solution increases in strength, the large gravity difference between it and the fiuoboric acid solution results in a layering of the two solutions at a point above the surface of the anode. When the stannous fluoborate solution reaches the desired strength, the unit is made continuous with the concentrated stannous fluoborate solution being continuously drawn off at the bottom of the cell while fiuoboric acid solution is added to the cathode area.

The bottom of the cathode container must be composed of a woven material (non-wettable by mercury) which has low electrical resistance, is relatively durable (i.e. does not decompose rapidly in fiuoboric acid solution) and has sufficient porosity to permit free transfer of the electrolyte (and the fluoborate ions contained therein). Generally speaking, the porosity of the woven material, i.e. cloth, should not be in excess of about 40 cu. ft. of air/minute per square foot of cloth under pressure of /2 inch of water. The cloth may be made of solid polymers of ethylene, propylene, acrylonitrile, etc.

When the cathode container is not provided with a porous Woven bottom, reduction of the metal to be converted into its fluoborate takes place from the electrolyte covering the surface of the mercury. Under these conditions, there is no apparent tendency of the metal to amalgamate with the mercury but rather to form a light spongy metal deposit on the surface of the mercury. This spongy mass quickly breaks away from the mercury surface and floats to the surface of the fiuoboric acid solution, thereby creating the problems encountered in prior art electrolytic operations.

Although the temperature in the electrolytic cell may vary within wide limits, it is preferred to maintain the main body of the electrolyte within the range of about to 140 F. Under these conditions, the temperature of the electrolyte in the immediate vicinity of the cathode is about to F.

The electrolytic cell employed for carrying out the process of this invention comprises a cathode near the top of the cell and an anode of the metal to be converted into its fluoborate near the bottom of the cell, said cathode comprising a pool of mercury in a container provided with a porous woven bottom.

The process and apparatus defined above are effective in producing substantially quantitative yields of metallic fluoborates. Moreover, use of mercury as cathode has the important advantage that the metal coming out does so as a dense amalgam with the mercury and not as a voluminous spongy metal deposit. The fact that there is no spongy coating of the cathode in the present invention is believed to be due to the fact that any metallic ions present in the solution pass through the porous woven bottom of the cathode container and are reduced in the mercury forming an amalgam. This amalgam is a liquid miscible With mercury at metal concentration of less than about 12%, and the metal is easily stripped out electrolytically in an auxiliary purification cell.

The process has the further advantage in that during operation hydrogen liberated at the mercury cathode comes off on the inside of the container so that mixing due to the violence of the bubbling of the gas on the surface is eliminated.

According to a specific embodiment of the present invention, process and apparatus are provided wherein the mercury cathode is continuously recirculated for purification. This operation is possible only where permanent materials (completely unaffected by the fiuoboric acid solution) having low electrical resistance are used as the porous woven bottom of the cathode container. The preferred permanent mate-rial is linear solid polyethylene. Linear solid polyethylene is a filament-forming material having a density'of atleast 0.94, typically 0.94 to 0.98.

Among other suitable materials is isotactic solid polypropylene w-hich, as known in the art, readily forms filaments. When the porous woven bottom is made of nonpermanent materials such as solid acrylonitrile polymer, the bottom must be changed at least about every twentyfour hours.

With use of permanent materials for the bottom of the cathode container, a completely continuous mercury cathode system may be employed wherein contaminated mercury of the cathode pool is continuously drawn off and allowed to drop into a small auxiliary electrolytic cell outside the main cell where it becomes an anode pool. Any metal in the mercury is removed electrolytically and plated out on a suitable cathode composed of carbon, tin, etc. Purified mercury is continuously pumped from the anode pool and passed through cold water which washes and cools the mercury. The cooled purified mercury is then continuously supplied to the cathode container in the main electrolytic cell.

More specifically, the contaminated mercury is placed in the auxiliary cell and is made the anode therein. With agitation, the metal is removed from the mercury and plated out on the cathode. The bath conditions are such that a hard, dense plate results so that both metal and mercury are recovered. Good agitation of the anode pool is necessary to prevent undesired mercurous oxide formation.

In commercial use of the continuous recirculating mercury cathode system, almost no attention has to be given to the apparatus other than replacing the metal anode every few weeks and removing the dense metal plate from the auxiliary purification cell.

The attached drawing is a schematic representation of apparatus suitable for carrying out continuous electrolytic production of stannous fiuoborate in accordance with the preferred modification of the present invention.

Referring to the drawing, there is shown a main electrolytic cell 1. The cell may be fabricated from iron or other suitable material. When the cell is made of a material which conducts electricity and/or is subject to corrosion by the contents of the cell, it may be provided with a plastic lining composed, for example, of polyethylene, polypropylene, polystyrene, unplasticized polyvinyl chloride, etc. Cell 1, having a suitable support 2, is provided with a cathode 3 comprising a container 4 having a pool of mercury 5 and cathode lead 6 immersed in the mercury. The cathode lead must have good conduction and may be any metal which does not amalgamate to substantial extent, erg. iron. The bottom 4a of the cathode container is a porous, woven material which is inert to fluoboric acid solution but offers low resistance to the electrolytic reaction. The material is composed preferably of linear solid polyethylene. The sides of the cathode container are relatively rigid and are made preferably of non-porous plastic material which is inert to fluoboric acid solution. Typical materials include polyethylene, polystyrene, polypropylene, polytetrafluoroethylene, metals coated with unplasticized polyvinylchloride, etc. An anode 7, composed of tin in granular, shot, stick or any other suitable form, is disposed near the bottom of cell 1 and is provided with anode lead 8. The anode lead can be a threaded carbon rod screwed into a carbon slab 8a at the bottom of the cell. Alternatively, the anode lead can be composed of any suitable metal provided with acid-resistant insulation, e.g. polyethylene.

In operation, fluobo-ric acid solution is introduced through line 9 to form the electrolyte in cell 1. The electrolyte entering the cell may be heated, and/or external heat may be applied to the cell to attain the desired temperature, i.e. about 120 to 140 F. The temperature of the electrolyte in the immediate vicinity of the cathode is generally about 130 to 140 F. Upon passing an electric current through the electrolyte, tin ions from the anode react with the fiuoboric acid solution to produce stannous fluoborate solution at the anode surface. Hydrogen is evolved at the cathode. As the stannous fluoborate solution increases in strength, the gravity difference between it and the fluoboric acid solution results in formation of a layer 11 of the stannous fiuoborate solution at the lower part of the cell. The fiuoboric acid solution forms a layer 12 above the stannous fiuoborate solution. The concentrated fluoborate solution is withdrawn through line 13 by means of pump 14, while fiuoboric acid solution is added to the cathode container via line 9. The fiuoborate solution is collected in receiver 15.

Mercury is continuously withdrawn from cathode container 4 via line 16 and permitted to flow into a small auxiliary electrolytic cell 17 fabricated from the same material used for cell 1, where the pool of mercury becomes an anode 18. The mercury has anode lead 19 immersed in it. Cell 17 is also provide-d with cathode plate 21, which is preferably carbon but may also be composed of any suitable metal such as tin. Plate 21 is provided with cathode lead 22. The cell contains a suitable electrolyte 20, e.g. fiuoboric acid solution. Agitation in the cell is provided by means of agitator 23 to prevent mercurous oxide formation. Upon application of electric current to cell 17, tin from the mercury is plated out on cathode plate 21. The amount of tin removed from the cathode mercury can be varied by changing the current on the power source, and the temperature of the cathode mercury can be controlled by changing the pumping rate. Purified mercury is continuously sent via line 24 by means of pump 25 into washing tower 26 in which cold water is continuously introduced through line 27 and withdrawn through line 28. The cooled purified mercury is returned via line 29 to cathode container 4 of the main electrolytic cell.

The following example of continuous production of stannous fiuoborate will serve to illustrate this invention.

Example Referring to the accompanying drawing, the main electrolytic cell was provided with a layer of 67-68% stannous fluoborate solution to a height of slightly above the surface of the tin anode. A solution of 50% fluoboric acid was introduced into the cell until the mercury cathode container was immersed. The cathode container was provided with a porous woven bottom composed of linear solid polyethylene cloth having a thread count per inch of 60 x 25 and a porosity of 4.2 cu. ft. of air/minute per square foot of cloth under pressure of /2'inch of water. The electric current was turned on and maintained at 90 amperes at 7-10 volts. A current density of 460 amps/sq. ft. on the cathode and approximately 86 amps/sq. ft. on the anode was established at the 90 ampere current. A 68% stannous fluoborate solution Was continuously pumped from the bottom of the cell at a rate of 0.1 gallon per hour. A solution of 50% fiuoboric acid was added at the same time to the cathode area at the rate of 0.117 gallon per hour.

Mercury was continuously withdrawn from the cathode container and allowed to flow into the auxiliary electrolytic cell to form a pool of mercury as anode. The auxiliary cell was supplied with a carbon cathode plate and a bath composed of 775 grams of water, 103 grams of 50% fluoboric acid solution and 510 grams of 47% stannous fluoborate solution. To this was added a solution of 7.5 grams of gelatin and 0.13 gram of beta naphthol in grams of water. The bath was maintained at temperature of about to F. An electric current of one ampere was applied to the cell, and the mercury was thus continuously purified. A dense hard plate of the tin present in the mercury formed on the cathode. Mercury from the pool was continuously pumped into a cold water wash, and the cooled purified mercury was continuously fed into the cathode container of the main electrolytic cell.

Since -many variations may be made in the process and apparatus without departing from the scope of the invention, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. An electrolytic cell for producing a metallic fluoborate containing fluoboric acid solution as electrolyte, a cathode near the top of the cell, said cathode comprising a pool of mercury in a container provided with a porous woven bottom composed of an inert material which has low resistance to the electrolytic reaction, an anode of the metal to be converted into its fluoborate near the bottom of the cell and means for withdrawing metallic fluoborate from the vicinity of the anode.

2. An electrolytic cell in accordance with claim 1 wherein the porous woven bottom is composed of linear solid polyethylene.

3. Apparatus for producing a metallic fluoborate which comprises a main electrolytic cell containing fluoboric acid solution as electrolyte, a cathode near the top of the cell, said cathode comprising a pool of mercury in a container provided with a porous woven bottom composed of linear solid polyethylene and an anode of the metal to be converted into its fluoborate near the bottom of the cell, and having associated therewith an auxiliary electrolytic cell, means for circulating mercury from the main electrolytic cell to the auxiliary electrolytic cell and from the auxiliary electrolytic cell to the main electrolytic cell, the auxiliary electrolytic cell containing the circulating mercury as anode and a cathode plate, and means for withdrawing metallic fluoborate from the vicinity of the anode of the main electrolytic cell.

4. A method for electrolytically producing a metallic fluoborate which comprises introducing fluoboric acid solution as electrolyte in an electrolytic cell provided with an anode of the metal to be converted into its fluoborate near the bottom of the cell and a mercury cathode near the top of the cell, the mercury cathode being maintained in a container provided with a porous woven bottom composed of an inert material which has low resistance to the electrolytic reaction, passing an electric current through said cell, thereby forming metallic fluoborate, and withdrawing metallic fluoborate from the vicinity of the anode.

5. The method of claim 4 in which the porous woven bottom of the cathode container is composed of linear solid polyethylene.

6. A continuous method for electrolytically producing a metallic fluoborate which comprises continuously introducing fluoboric acid solution as electrolyte in a main electrolytic cell provided with an anode of the metal to be converted into its fluoborate near the bottom of the cell and a mercury cathode near the top of the cell, the mercury cathode being maintained in a container provided with a porous woven bottom composed of linear solid polyethylene, passing an electric current through said cell, thereby forming metallic fluoborate, continuously withdrawing contaminated mercury from the cathode container and passing it to an auxiliary electrolytic cell where the mercury becomes an anode, said auxiliary electrolytic cell being provided with a cathode plate on which metal present in the mercury is plated out, continuously returning the purified mercury to the cathode container of the main electrolytic cell and continuously withdrawing metallic fluoborate from the vicinity of the anode of the main electrolytic cell.

7. The method of claim 6 in which the anode is tin and stannous fluoborate is produced.

References Cited by the Examiner UNITED STATES PATENTS 1,200,025 10/1916 Reed 204220 1,368,955 2/ 1921 Matsushima 204251 2,150,775 3/1939 Messner 204251 2,673,837 3/1954 Lowe et al. 20494 2,681,320 6/1954 Bodamer 204-296 2,739,869 3/ 1956 Parsons 204-296 2,772,228 11/1956 McElroy et al.

3,065,163 11/1962 Honsberg 204-220 3,074,861 1/ 1963 Avery 204-220 JOHN H. MACK, Primary Examiner.

L. WIs LQ RNOY,

Assistant Examiners, 

4. A METHOD FOR ELECTROLYTICALLY PRODUCING A METALLIC FLUOBORATE WHICH COMPRISES INTRODUCING FLUOBORIC ACID SOLUTION AS ELECTROLYTE IN AN ELECTROLYTIC CELL PROVIDED WITH AN ANODE OF THE METAL TO BE CONVERTED INTO ITS FLUOBORATE NEAR THE BOTTOM OF THE CELL AND A MERCURY CATHODE NEAR THE TOP OF THE CELL, THE MERCURY CATHODE BEING MAINTAINED IN A CONTAINER PROVIDED WITH A POROUS WOVEN BOTTOM COMPOSED OF AN INERT MATERIAL WHICH HAS LOW RESISTANCE TO THE ELECTROLYTIC REACTION, PASSING AN ELECTRIC CURRENT THROUGH SAID CELL, THEREBY FORMING METALLIC FLUOBORATE, AND WITHDRAWING METALLIC FLUOBORATE FROM THE VICINITY OF THE ANODE. 